fix failing sanity check for GCCcore 5.x + sanity check under --module-only for all GCCcore versions

[boegel]

The changes in #2033 broken the sanity check for GCCcore-5.3.0.eb, GCCcore-5.4.0.eb, GCCcore-5.5.0.eb (while older versions like GCCcore-4.9.3.eb and newer versions are still fine):

== sanity checking...                                                                                                                                                                                         ...
  >> file 'libexec/gcc/x86_64-pc-linux-gnu/5.4.0/lto1' or 'lib/gcc/x86_64-pc-linux-gnu/5.4.0/lto1' found: FAILED
  >> file 'libexec/gcc/x86_64-pc-linux-gnu/5.4.0/lto-wrapper' or 'lib/gcc/x86_64-pc-linux-gnu/5.4.0/lto-wrapper' found: FAILED
  >> file 'libexec/gcc/x86_64-pc-linux-gnu/5.4.0/liblto_plugin.so' or 'lib/gcc/x86_64-pc-linux-gnu/5.4.0/liblto_plugin.so' found: FAILED
...
  >> (non-empty) directory 'lib/gcc/x86_64-pc-linux-gnu/5.4.0' found: FAILED                                                                                                                                 

The issue is that even though the updated config.guess suggests that the x86_64-pc-linux-gnu should be used, the GCCcore installation still has x86_64-unknown-linux-gnu.

In addition, it made the problem with --module-only worse, since rather than just having a failing sanity check (see #1588), the GCC easyblock now produces a hard crash because self.platform_lib is never defined when using --module-only:

$ eb GCCcore-5.4.0.eb --module-only
== sanity checking...
ERROR: Traceback (most recent call last):
  File "/lib/python2.7/site-packages/easybuild_framework-4.2.1.dev0-py2.7.egg/easybuild/main.py", line 114, in build_and_install_software
    (ec_res['success'], app_log, err) = build_and_install_one(ec, init_env)
  File "/lib/python2.7/site-packages/easybuild_framework-4.2.1.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 3234, in build_and_install_one
    result = app.run_all_steps(run_test_cases=run_test_cases)
  File "/lib/python2.7/site-packages/easybuild_framework-4.2.1.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 3139, in run_all_steps
    self.run_step(step_name, step_methods)
  File "/lib/python2.7/site-packages/easybuild_framework-4.2.1.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 2994, in run_step
    step_method(self)()
  File "/lib/python2.7/site-packages/easybuild_easyblocks-4.2.1.dev0-py2.7.egg/easybuild/easyblocks/g/gcc.py", line 635, in sanity_check_step
    common_infix = os.path.join('gcc', self.platform_lib, self.version)
AttributeError: 'EB_GCC' object has no attribute 'platform_lib'

I've fixed both issues by slightly loosening up the sanity check in the GCC easyblock w.r.t. the "configuration name" subdirectory, which was already the proposed solution in #1588 .

We don't really care what this directory is called, just that it's there.

This allows us to drop the use of self.platform_lib entirely.

The updated config.guess (result of the changes in #2033) is still used by the GCC build process internally, so that's basically left untouched (we just don't use config.guess ourselves anymore now).

fixes #1588

[boegel]

This makes eb --module-only works for all 22 GCCcore easyconfigs we have.

[verdurin]

Looks fine. Tested by demonstrating it would re-instate an old GCCcore module.

[verdurin]

Going in, thanks @boegel!